In a typical tensioner for maintaining tension in an engine timing chain, a plunger for exerting the tensioning force protrudes from a plunger-accommodating hole in a tensioner housing, and is slidable in the plunger-accommodating hole. The plunger is biased in the protruding direction by a plunger-biasing spring and by hydraulic pressure exerted by oil in a high pressure oil chamber formed by the plunger and the tensioner housing.
FIG. 13 shows a typical prior art tensioner 500, referred to as a “ratchet type tensioner.” A plunger 520 is slidable in, and protrudes from, a plunger-accommodating hole 512 in a housing 510. The plunger-accommodating hole is supplied with oil under pressure through an oil supply port 511 from an oil pump or other source of oil under pressure. The plunger 520 is biased by a spring in the protruding direction such that it can apply tension to a traveling timing chain. A ratchet mechanism comprises a pawl 530 pivoted on the housing 510 and a set of a ratchet teeth 521 formed as a rack of teeth along the plunger 520. A check valve allows oil to flow through the oil supply port 511 into the high pressure oil chamber formed by the plunger and the plunger-accommodating hole, but blocks reverse flow of oil as disclosed in United States Patent Application Publication 2003/0134703, published on Jul. 17, 2003.
In the ratchet-type tensioner 500 the teeth of pawl 530 are pressed against the ratchet teeth 521 by a pawl-biasing spring 540, which allows the plunger 520 to advance when the timing chain becomes slack, but limits retracting movement of the plunger by exerting a “wedge” effect on the ratchet teeth 521. Limiting the retracting movement of the plunger becomes important to avoid “flip-flop” noises that occur upon engine start-up when oil in the high pressure oil chamber is depleted, as is often the case when the engine is first started after having been out of operation for a long time.
The prior art ratchet-type tensioner 500 can reduce flip-flop noises if the plunger backlash, i.e., the amount of retracting movement allowed by the ratchet mechanism, is small. However, in some cases, when the plunger has protruded excessively, the ratchet pawl can cause the plunger 520 to remain in the excessively protruded condition and thereby apply an excessive force to the timing chain, causing a so-called “beat note” or “whirring” noise.
Another problem is that the plunger 520 in tensioner 500 can seize, causing excessive tension in the timing chain over a long time.
In the tensioner 500, the ratchet teeth 521 are formed along a narrow elongated part of the outer circumference of the plunger. A slight rotation of the plunger will prevent the teeth of the pawl from engaging the ratchet teeth uniformly over the full width of the pawl teeth. The resulting eccentric tooth engagement can result in excessive loads causing chipping of the ratchet teeth or the teeth of the pawl. Such chipping can quickly lead to failure of the ratchet mechanism.